High dynamic radiation detection device

ABSTRACT

A device for detecting energy radiation includes a semiconducting material capable of converting the radiation into electric charges, a measurement electrode, and a measurement circuit to measure the current delivered by the electrode. The device further includes polarisation electrodes constituted of conductive zones insulated electrically from each other, the polarisation electrodes and a measurement electrode sandwiching the material, and a voltage supply adapted to bring each of these conductive zones to an adjustable voltage.

“This application is a national phase of PCT/FR00/01348 which was filedon May 18, 2000, and was not published in English.”

TECHNICAL FIELD

The invention relates to a radiation detection device with high dynamicrange. By high dynamic range detection device one means a device capableof detecting both low flux and high flux radiation.

The radiation in question can be X or γ radiation, but one can also useother types of radiation, of corpuscular type for example, such asproton beams. The only restriction is that this radiation must be ableto create electric charges within a semiconducting material in a volumeof the order of a cubic millimetre.

The invention can be applied in particular in the medical domain. Forexample, for radiography, the X-rays used, before arriving on thedetector device, cross the body of a patient or are absorbed innon-homogeneous fashion. The exiting flux can thus vary considerablylocally (several decades).

Another example can be taken from radiotherapy: the patient can beirradiated at very low level to begin with for positioning, and thenvery highly irradiated afterwards for treatment.

A further domain is that of non-destructive monitoring by radiography,for example inside containers (loading ships) which can have very variedabsorption levels.

STATE OF PRIOR ART

At present, and in particular in the medical domain, radiation detectors(for example X or γ) are usually scintillation detectors operating on aprincipal of indirect detection: the incident photon reacts with thescintillation substance, creating photons of another type, photons whichare multiplied by a photo-multiplier in order to provide a measurableelectric signal.

These detectors have an efficiency and a resolution which can beinsufficient for certain applications.

These characteristics can be improved by replacing the scintillationdetectors by semiconductor detectors. FIG. 1 is a diagram of a detectorof this type.

As shown, the detector comprises a semiconducting material 2 sandwichedby two electrodes 4 and 6, supply means 8 capable of bringing theelectrode 6 to an appropriate voltage (−V), means for measuring thecurrent (i) delivered, comprising, in the example shown, an amplifier 10whose output is returned to the input by a condenser 12, acircuit-breaker 14 also being attached to the terminals of thiscondenser. The device also comprises an apparatus for measuring thecurrent (or the voltage 16). The radiation one wishes to detect isreferenced 20 and crosses the semiconducting material 2.

The operation of this device is as follows. The radiation 20 interactswith the semiconducting material 2, creating electric charges. For thesame incident radiation, the number of charges created is of an order ofsize greater than that obtained by indirect detection by a scintillationdetector. The electric field created in the material by the electrodesmakes it possible to collect these charges on the electrodes and inparticular on electrode 4. These charges are then stored in thecondenser 12 and processed in the circuit 16 which delivers a signalrepresentative of the radiation.

The integration condenser 12 has dimensions enabling it to store themaximum quantity of charges that the semiconducting material candeliver. This quantity is a function of the value of the incident flux.If the flux is very high, the number of charges to store will be highand the capacities needed for storing them will he high. These capacityvalues may either not be available on the market for electroniccomponents, or may involve integrated capacity surfaces that are too bigfor the space available on the circuit.

Thus the solution lies in reducing the quantity of charges to be stored,only when the incident flux is too high. But it is essential to preservethe fundamental qualities of the detector, that is to say good spatialresolution or good contrast. The spatial resolution is the minimumdistance separating the points of interaction of two incident photonswith the material in order that the detector can differentiate betweenthem.

The quantity of charges to be stored cannot be diminished by simplereduction of the number of photons arriving at the detector. Thismethod, valid for high fluxes, is not suitable for low fluxes and thedetector would lose the dynamic range needed for the application.Furthermore, the number of incident photons must remain consistent sincethe noise of the electric signal is proportional to 1N where N is thenumber of photons absorbed by the whole volume of the detector.

The present invention has the particular aim of proposing a radiationdetection device with high dynamic range, which does not have thesedrawbacks and makes it possible to satisfy these contradictoryrequirements.

DESCRIPTION OF THE INVENTION

In order to do this, the invention proposes a device whose essentialcharacteristic is that the polarisation electrode, unlike themeasurement electrode, is fragmented into conductive zones insulatedfrom each other electrically, the supply means being capable of bringingeach of these zones to an appropriate voltage.

During operation, the incident radiation is injected in a direction,perpendicular to the fractionation direction of the polarisationelectrode.

These means make it possible to operate the device in two differentways, according to the applications envisaged:

either when working at low voltage, on the totality of the material, inwhich case low noise is obtained and the contrast is improved,

or when working on only a part of the material but at high voltage, inwhich case the spatial resolution is improved.

To be precise, the aim of invention is thus a device for detection ofenergy radiation, comprising a semiconducting material able to convertthis radiation into electric charges, a measurement electrode and ameasurement circuit for measuring the current delivered by thiselectrode, characterised in that it further comprises polarisationelectrodes constituted of conductive zones insulated from each otherelectrically, said polarisation electrodes and the measurement electrodesandwiching the material and the supply means capable of bringing eachof these conductive zones to an adjustable appropriate voltage.

Preferably, the semiconducting material takes the form of aparallelepiped bar with a depth intended to be oriented parallel to thedirection of the radiation, a width and a height, this bar having twoparallel faces separated by said height, the measurement electrode andthe polarisation electrodes being set on these faces.

Preferably, in this variant, the conductive zones of the polarisationelectrode are in the form of rectangular strips with a length parallelto the width of the bar and a width parallel to the depth of the bar.

In another variant, the measurement electrode is constituted ofrectangular conductive strips with a length parallel to the depth of thebar and a width parallel to the width of the bar.

The fragmentation zones can take various forms, rectangular strips inparticular.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, described above, shows a prior art detector;

FIG. 2 shows the evolution of the number of charges collected infunction of the polarisation value of the electrodes;

FIG. 3 is a diagram of the fragmentation of an electrode;

FIG. 4 shows an embodiment of the invention with zones of increasingwidth;

FIG. 5 shows another embodiment of the invention with a succession ofstrips of the same width;

FIG. 6 shows an embodiment of the invention in which the twopolarisation electrodes and the measurement electrode are fragmented intwo orthogonal directions.

DESCRIPTION OF PARTICULAR EMBODIMENTS

According to a first embodiment of the device according to theinvention, the number of charges collected is reduced by reducing theelectric field applied to the semiconducting material, thus the voltageapplied to the electrodes. This embodiment has never been used in priorart, because it was considered that by lowering the operational voltage,the charge carriers would migrate more slowly and would partly becaught. Depending on the location of the radiation-material interaction(near the electrodes or at the centre of the bar), the quantity ofcharges stored in the condenser will vary considerably and the measuredsignal will be difficult to reproduce. However, the Applicant hasdemonstrated that this does not concern high energy photons (of theorder of MeV). In fact, such photons create electron-hole pairs in alarge volume, with typical cross section of the order of 1 to severalmm². Thus charges exist throughout the whole volume separating theelectrodes. When the number of incident photons is high (which is thecase when the problem of excess charges arises), it can be consideredthat the division of charges in the volume of the semiconductor isuniform (on average). Therefore, although the output signal reduces withthe polarisation voltage, it nonetheless remains stable andreproducible. This is confirmed in FIG. 2, which shows the measuredcharge C (arbitrary unit) as a function of the polarisation voltage V,expressed in volts. The curve (exponential) is smooth and demonstrates adirect relationship between the variation in voltage and the variationin the quantity of charges collected.

This first embodiment thus makes it possible to reduce the quantity ofcharges to be processed while still preserving the contrast. But this isdetrimental to the spatial resolution. In order to counteract thisinconvenience, the invention proposes another embodiment (which moreovercan be associated with the first) and which uses the fragmentation ofthe polarisation electrode opposed to the measurement electrode.

FIGS. 3, 4 and 5 show examples of embodiments of this fragmentation.These examples refer to a parallelepiped bar but, evidently, this shapeis not the only one possible. In FIG. 3, the bar 2 can be seen on alarge scale, labelled letter p, parallel to the direction 20 of theradiation to be detected; the bar has a width l and a height h. Thefaces separated by the height h carry the electrodes. In the caseillustrated, there are three polarisation electrodes 6A, 6B and 6C,insulated electrically from each other and linked to three supply means8A, 8B, 8C, which are able to supply three adjustable voltages to thesethree electrodes 6A, 6B, 6C, independent from each other. As for themeasurement electrode 4 it is linked to a single circuit for measuringcurrent, that is in the example shown, a circuit comprising an amplifier10, a condenser 12, a circuit breaker 14 and a circuit for measuring thecurrent 16.

Thus one obtains a device whose interaction volume is adjustableaccording to whether or not one applies voltages to the differentpolarisation electrodes.

Depending on the value of the incident flux, one or several zones can beconnected and thus activated for collecting charges; other zones canhave zero polarisation. When the incident flux is low, all thepolarisation electrodes can be connected in order to collect the maximumof charges; when the flux rises, one or several electrodes can bedeactivated, earthed for example.

In this operational mode, only the charges created regarding thesupplied polarisation electrodes are collected (by the singlemeasurement circuit). Since the number of charges to be collected islower, the voltage can be adjusted to its optimum value, that is to avalue sufficiently high to obtain good quality spatial resolution.

Besides, since the volume of material involved is smaller, the number ofphotons participating in the creation of charges is smaller, whichresults in a rise in the noise level, thus lowering the contrast. Thisoperational mode thus makes it possible reduce the quantity of chargeswithout losing spatial resolution but to the detriment of the contrast.

In certain cases, it may be suitable to use the two operational modesdescribed at the same time, by reducing the surface of the activeelectrodes and applying a voltage to them which is lower than theoptimum voltage.

FIG. 4 illustrates a variant of the device intended to respond to fluxwith a very high dynamic range. In this variant, which again comprisesthree polarisation electrodes 6A, 6B and 6C, under the form of strips,the width of the conductive strips (counted parallel to the depth of thebar) changes from one strip to the other following a common ratio of 10(the width is 10 times greater for zone 6B than for zone 6A and 10 timesgreater for zone 6C than for zone 6B). Thus one can obtain two decades.The length of the strips 6A, 6B, 6C counted parallel to the width l ofthe bar is closely equal to the width of the bar.

The first zone 6A, very narrow, is to be used with an optimum voltageand will make it possible to obtain information with high spatialresolution, whereas the second zone 6B and possibly the third 6C will beused with a low voltage and will make it possible to obtain informationwith high contrast.

This variant is interesting in the case of beams, which diffuse into thematerial in a pear-shaped volume, the depth of the narrow part of thepear fixing the width of the first zone 6A.

This variant can also be used in the case of an incident multi-energybeam, for example with energy varying from 50 keV to 20 MeV. The firstelectrode will be dimensioned to stop 99% of the photons of 50 keV (lowenergy), the second to stop 95% of the photons of several hundreds ofkeV and the third to stop the an remaining high energy photons (over 500keV). Typically, the electrodes will have respective widths of 50 to 100μm, from 200 to 500 μm and from 2 to 3 cm.

This system makes it possible to use a single detector and a singleelectronic measurement circuit to produce three images at differentenergies. After processing the images, it is possible to deduce thenature of the substances that have attenuated the beam.

FIG. 5 illustrates another case where six electrodes 6A, 6B, 6C, 6D, 6E,6F are constituted of strips of the same width. Thus one obtains aperiodic structure. The width of the strips can, for example, be 10 mm.

The device according to the invention is particularly advantageous whenit is repeated a certain number of times to constitute a matrix. Thedevice is then capable of determining the place of interaction of aphoton with the semiconducting material: it then makes it possible toproduce an image. Since the fragmentation of the electrodes does notincrease the dimensions of the device significantly, it can therefore beset next to other devices of the same type.

Advantageously, to avoid cutting out and manipulating thin devices,stamping can be carried out on a single bar but of greater width, with alower fragmentation of the electrode in the depth direction. This isshown in FIG. 6. In this figure, a device can be seen which comprisestwo polarisation electrodes 6A and 6B and six measurement electrodes 4a, 4 b, 4 c, 4 d, 4 e, 4 f, constituting rectangular strips parallel tothe depth of the bar.

The semiconducting material suitable for use in the invention describedabove can be one of several and, advantageously, can be chosen among thesemiconductors with high resistivity (typically higher than 10⁷ Ωcm) soas to provide a low current when it is polarised and non lit, and amongthe semiconductors with a factor μτ (product of the mobility by thelifetime of the carriers) sufficiently high so that the charges createdand submitted to an electric field have the time to reach the electrodes(typically for an electric field of 0.1 V/cm, μ=100 cm²s⁻¹v⁻¹, τ=1μsec).

In particular, the material can be chosen from among the groupconsisting of CdTe, CdZnTe, AsGa, PbI₂, HgI₂ and Se.

In the case of detection of X-rays of several MeV for radiography, thedevice according to the invention is advantageously used under the formof a parallelepiped of CdTe with an input surface of the order of 1 mm²,the depth of the bar being 60 mm, to provide a stopping power sufficientfor photons of this energy.

What is claimed is:
 1. Device for detecting energy radiation, comprisinga semiconducting material capable of converting the radiation intoelectric charges, a measurement electrode and a measurement circuit tomeasure the current delivered by the electrode, said device furthercomprising: polarisation electrodes constituted of conductive zonesinsulated electrically from each other, said polarisation electrodes anda measurement electrode sandwiching the material; and a voltage supplyadapted to bring each of these conductive zones to an adjustablevoltage.
 2. Device according to claim 1, wherein the semiconductingmaterial is present in the form of a bar with a depth, a width and aheight, this bar having two parallel faces separated by said height, themeasurement electrode and the polarisation electrodes being set on thesefaces.
 3. Device according to claim 2, wherein said bar has aparallelepipedic form.
 4. Device according to claim 1, wherein thesemiconducting material is chosen from among the group consisting ofCdTe, CdZnTe, AsGa, PbI₂, HgI₂ and Se.
 5. Device according to claim 1,wherein said voltage supply is further adapted to bring each of saidconductive zones to an adjustable voltage independently from each other.6. Device for detecting energy radiation, comprising: a semiconductingmaterial capable of converting the radiation into electric charges, saidsemiconducting material being present in the form of a bar with a depth,a width, and a height, this bar having two parallel faces separated bysaid height; a measurement electrode and a measurement circuit tomeasure the current delivered by the electrode; polarisation electrodesconstituted of conductive zones insulated electrically from each other,said polarisation electrodes and a measurement electrode sandwiching thematerial, being set on said parallel faces, conductive zones of saidpolarisation electrodes being in the form of rectangular stripes havinga length parallel to the width of said bar and a width parallel to thedepth of said bar; and a voltage supply adapted to bring each of theseconductive zones to an adjustable voltage.
 7. Device according to claim6, wherein all the strips have the same width.
 8. Device according toclaim 6, wherein the strips have an increasing width in the direction ofthe depth of the bar.
 9. Device according to claim 8, wherein the stripshave widths with a common ratio of
 10. 10. Device according to claim 6,wherein the measurement electrode is constituted of rectangularconductive strips having a length parallel to the depth of the bar and awidth parallel to the width of the bar.
 11. Device according to claim 6,wherein the semiconducting material is chosen from among the groupconsisting of CdTe, CdZnTe, AsGa, PbI₂, HgI₂ and Se.
 12. Deviceaccording to claim 6, wherein said voltage supply is further adapted tobring each of said conductive zones to an adjustable voltageindependently from each other.